Abstract

A comparative vibrational spectroscopic study of ferroelectric TAAP, its deuterated analogue (DTAAP) and non-ferroelectric TADP has been carried out. The similarities and dissimilarities between the spectra of the three crystals are discussed. The temperature variation of the Raman spectra of TAAP and DTAAP across the phase transition clearly suggests an order-disorder model for the phase transition in these crystals and gives no evidence for a displacive model. The vibrations of the $PO_4$ groups and hydrogen bonds show significant variations in line width or shape across the transition temperature indicating that the cooperative phenomenon involving $PO_4$ groups and hydrogen bonds is responsible for the phase transition.